Mammalian hairs are formed by the differentiation and keratinization of cells produced in the epidermal matrix which invests the dermal papilla at the base of the hair follicle. The importance of the dermal papilla, a discrete population of specialized fibroblasts, in stimulating and controlling the growth of hair has been established in a series of experimental studies using the rat vibrissa follicle, a system first employed by Cohen, J., (1961), J. Embryol. exp. Morph., 9, 117-127. Oliver, R. F., (1966), J. Embryol. exp. Morph., 15, 331-347 and Oliver, R. F., (1966), J. Embryol. exp. Morph., 16, 231-244, demonstrated that following removal of the dermal papilla, hair growth ceases and it is not resumed until a new dermal papilla regenerates from dermal sheath cells. Transplantation studies have shown that dermal papillae can induce hair growth when implanted into vibrissa follicles (Oliver, R. F., 1967, J. Embryol. exp. Morph. 18, 43-51; and Ibrahim, L. and E. Wright, 1977, Nature, Lond. 265, 733-734) and can interact with ear and scrotal sac epidermis to induce the formation of new follicles and the subsequent growth of hair fibers (Oliver, R. F., 1970, J. Embryol. exp. Morph., 23, 219-236). Wounding of the dermal papilla can lead to producation of longer hairs associated with extended growth cycles (Jahoda, C. A. B. and R. F. Oliver, 1984, J. Embryol. exp. Morph., 79, 211-224; and Jahoda, C. A. B. and R. F. Oliver, 1984, J. Embryol, esp. Morph., 83, 81-93).
Microsurgical removal of the vibrissa follicle dermal papilla has allowed the growth of this discrete population of cells in culture (Jahoda, C. A. B and R. F. Oliver, 1981, Br. J. Dermatol., 105, 623-627. When compared with skin fibroblasts, cultured dermal papilla cells display unique morphological and behavioral characteristics and, in particular, form discrete cell aggregates when they approach confluence (Jahoda, C. A. B. & R. F. Oliver, 1984, J. Embryol. exp. Morph., 79, 211-224). Cultured papilla cells of early passage numbers have also been shown to retain the ability to induce the development of new, hair producing bulbs when implanted as pellets into the bases of amputated follicles (Jahoda, C. A. B., K. A. Horne & R. F. Oliver, 1984, Nature, Lond., 311, 560-562; and Horne, K. A., C. A. B. Jahoda & R. F. Oliver, 1986, J. Embryol. exp. Morph., 97, 111-124. These experiments clearly demonstrate the importance of the dermal papilla in the induction and maintenance of hair growth. Two broad classes of interactions can be envisaged involving the dermal papilla and surrounding follicular tissue namely: (1) direct intercellular interactions involving cell contact and (2) bi-directional interactions mediated by diffusible factors. An important class of diffusible effector molecules are the so-called polypeptide growth factors (e.g., hair growth promoters) that can be detected in vitro by their ability to modulate DNA synthesis in appropriate target cells (Sporn, M. B. and A. B. Roberts, 1988, Nature, Lond., 332, 217-219).